Cell Division Final[1]

7/30/2019 Cell Division Final[1]

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CELL DIVISION



Cell division is a fundamental process

All cells come from pre-existing cells

It is necessary to replace worn out cells in multicellularorganisms

It is required for growth in multicellular organisms An increase in size will require an increase in surface area to

volume ration

Cell division subdivides the cytoplasm into small units (cells)surrounded by plasma membranes

It is necessary for reproduction in unicellular ormulticellular organisms



Cell division and reproduction

It is necessary for reproduction in unicellular

or multi-cellular organisms



Prokaryotes

Prokaryotes have no nucleus

They have a single circular

chromosome Prokaryotes simply divide their cells in

two by binary fission



Eukaryotes

Eukaryotes must divide their nucleus (and

other organelles such as mitochondria) in

preparation for cell division (mitosis or meiosis)

Before the nucleus divides the genetic material

replicates (duplicates)



Mitosis

Mitotic division results in genetically

identical eukaryotic cells ( a clone )

Mitosis is the basis of asexual

reproduction



Meiosis

Meiosis results in a halving of the chromosome

number in preparation for fertilisation

Meiosis shuffles genes in new combinations

Meiosis results in genetically different cells

Meiosis and fertilisation are the basis of sexual

reproduction



Mitosis

Mitosis is the process by which mostcells in plants and animals arereplicated.





Prophase

• Chromosomes condense

• Spindle apparatus forms; nuclear membrane

breaks up

•Centriole pairs move to poles



Metaphase

• Transition period; microtubules from centrioles penetrate nuclear region;

• complete formation of spindle apparatus by attaching to sister chromatids

•Metaphase is complete when all chromosomes are lined up at the cell’s equator



Anaphase

•Sister chromatidsseparate moving towards poles

•Independent daughter chromosomes



Telophase

•Chromosomes arrive at the poles

•Nuclear envelope forms from fusion of vesicles

•Chromosomes decondense



Cytoplasmic division

Known as cytokinesis

Final stage of cell division The cytoplasm of a parent cell

contains enzymes, organelles, and other operating machinery. When a daughter cell inherits what looks like a

blob of cytoplasm, it is really getting start-up machinery,

which keeps it operating until it can use its inherited DNA

for growing and developing on its own.



Results of Mitosis

• Two daughter nuclei

Each with same chromosome number as parent cell

Chromosomes in unduplicated form



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Meiosis and genetic

variation



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Genome

Genome: Complete complement of an

organism’s DNA. Includes genes (control traits) and non-coding

DNA organized in chromosomes.



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Genes

Eukaryotic DNA is organized

in chromosomes. Genes have specific places on

chromosomes.



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Heredity

Heredity – way of transferring geneticinformation to offspring

Chromosome theory of

heredity: chromosomescarry genes.

Gene – “unit of heredity”.



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Reproduction Asexual

Many single-celled organisms reproduce by splitting, budding, parthenogenesis.

Some multicellular organisms can reproduce

asexually, produce clones ( offspring genetically

identical to parent ).



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Sexual reproduction

Fusion of two gametes to produce a single

zygote.

Introduces greater genetic variation, allows

genetic recombination.

With exception of self-fertilizing organisms (e.g.

some plants), zygote has gametes from two

different parents.



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Chromosomes

Karyotype:

ordered display of an individual’s chromosomes.

Collection of chromosomes from mitotic cells.

Staining can reveal visible band patterns, gross

anomalies.



Meiosis KM 24

Karyotyping



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Homologues

Chromosomes exist in homologous pairs indiploid cells.

Exception: Sex chromosomes (X, Y).

Other chromosomes are known as autosomes, they have

homologues.



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In humans …

23 chromosomes donated by each parent (total = 46or 23 pairs).

Gametes (sperm/ova):

Contain 22 autosomes and 1 sex chromosome. Are haploid (haploid number “n” = 23 in humans ).

Fertilization/syngamy results in zygote with 2

haploid sets of chromosomes - now diploid. Diploid cell; 2n = 46 . ( n=23 in humans )

Most cells in the body produced by mitosis.

Only gametes are produced by meiosis.



Meiosis KM 30

Chromosome numbers

All are even numbers – diploid (2n) sets of

homologous

chromosomes!

Ploidy = number of

copies of each

chromosome.

Diploidy

M i i k diff f



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Meiosis – key differences from

mitosis Meiosis reduces the number of chromosomes by half . Daughter cells differ from parent, and each other.

Meiosis involves two divisions, Mitosis only one.

Meiosis I involves:

Synapsis – homologous chromosomes pair up. Chiasmata form ( crossing over of non-sister chromatids).

In Metaphase I, homologous pairs line up at metaphase plate.

In Anaphase I, sister chromatids do NOT separate .

Overall, separation of homologous pairs of chromosomes,rather than sister chromatids of individual chromosome.



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M i i 1



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Meiosis 1

First division of meiosis

Prophase 1: Each chromosome dupicates andremains closely associated. These are called sisterchromatids. Crossing-over can occur during thelatter part of this stage.

Metaphase 1: Homologous chromosomes align atthe equatorial plate.

Anaphase 1: Homologous pairs separate with sister

chromatids remaining together. Telophase 1: Two daughter cells are formed with

each daughter containing only one chromosome of the homologous pair.

M i i II

http://www.accessexcellence.org/AB/GG/comeiosis.html






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Meiosis II

Second division of meiosis: Gamete formation

Prophase 2: DNA does not replicate.

Metaphase 2: Chromosomes align at the

equatorial plate.

Anaphase 2: Centromeres divide and sister

chromatids migrate separately to each pole.

Telophase 2: Cell division is complete. Fourhaploid daughter cells are obtained.



Meiosis KM 38

Mitosis vs. meiosis



Meiosis KM 39

M i i i



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Meiosis creates genetic

variation During normal cell growth, mitosis produces

daughter cells identical to parent cell (2n to 2n)

Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossingover.

No daughter cells formed during meiosis aregenetically identical to either mother or father

During sexual reproduction, fusion of theunique haploid gametes produces truly uniqueoffspring.



Meiosis KM 41

Independent assortment



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Independent assortment

Number of combinations: 2n

e.g. 2 chromosomes in haploid

2n = 4; n = 2

2n = 22 = 4 possible combinations



Meiosis KM 43

In humans

e.g. 23 chromosomes in haploid

2n = 46; n = 23

2n = 223 = ~ 8 million possible combinations!





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Harlequin chromosomes



Meiosis KM 46

Random fertilization

At least 8 million combinations from Mom, and

another 8 million from Dad …

>64 trillion combinations for a diploid zygote!!!



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Meiosis & sexual life cycles Life cycle = sequence of

stages in organisms

reproductive history;

conception to

reproduction. Somatic cells = any cell

other than gametes, most

of the cells in the body.

Gametes produced by

meiosis.Generalized animal life cycle



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Sex is costly!

Large amounts of energy required to find a mate

and do the mating: specialized structures and

behavior required

Intimate contact provides route for infection by

parasites (AIDS, syphillis, etc.) Genetic costs: in sex, we pass on only half of genes

to offspring.

Males are an expensive luxury - in most species they contribute little to rearing offspring.

But



But …

More genetic diversity: more potential for survival of

species when environmental conditions change. Shuffling of genes in meiosis

Crossing-over in meiosis

Fertilization: combines genes from 2 separate individuals

DNA back-up and repair. Asexual organisms don't have back-up copies of genes, sexual

organisms have 2 sets of chromosomes and one can act as aback-up if the other is damaged.

Sexual mechanisms, especially recombination, are used to repairdamaged DNA - the undamaged chromosome acts as atemplate and eventually both chromosomes end up with thecorrect gene.

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Cell Division Final[1]